Eyelink: Improve error handling in Eyelink('Initialize').

Provide meaningful error message with abort if open_eyelink_system()
fails, instead of silent failure and confusing status return codes.
-> Minor problem, no need to rebuild Eyelink mex file. Will be folded
into release at next real rebuild.
New demo TurnTableDemo - Free your inner DJ!

A demo which uses PsychPortAudio's functions to simulate a good'ol turn
table for some recreational scratching and other auditory fun
activities.

Work in progress - This is a backup commit of a basic working state.